The amount of fat that your lamb has surrounding the outside and marbled through the middle will determine the cooking time and temperature you use.
For a lean piece of meat, cook at 230 degrees C / Gas 7-8 for the first 15 minutes, then turn the temperature down to 180 degrees C / Gas 4 to continue. The lamb should take about 25 minutes per 500g to cook to medium rare.
Using a hot oven in this manner will allow joints with less fat to get nicely browned on the outside before they become overcooked and dry in the middle.
For a roast with more fat, cook at 160 degrees C / Gas 2-3 for a longer period of time, allowing the fat to slowly melt and bathe the joint in its own juices. Meat cooked with this method will take about 30 minutes per 500g to reach medium rare.
To be exact...
You can also tell the temperature of your lamb using a meat thermometer. Some cooks prefer this, as it takes the guesswork out of roasting. Follow these temperatures for lamb based on your preference:
Rare: 42 degrees C
Medium-rare: 58 degrees C
Medium-well: 68 degrees C